This invention relates to a biaxially or multi-axially oriented, heat set film made from thermoplastic materials; the surface of this film is uneven, having many fine protrusions with an indentation around each one.
Biaxially or multiaxially oriented polyester films, for example biaxially oriented polyethylene terephthalate film, have good tensile strength, tear resistance, modulus of elasticity, transparency, and chemical and thermal resistance. Due to these properties, polyester film has found wide use in a variety of applications, including video, audio, and computer film substrates, as well as in packaging and reprographic products.
Polyester films must meet specific requirements for various applications. One such requirement is a sufficiently low coefficient of friction on the film surface. The rougher the film surface is, the better is the winding behavior and therefore the production yield. In addition to a low coefficient of friction and good mechanical properties, a polyester film for magnetic tapes must additionally possess a uniform surface roughness, good abrasion resistance, and dimensional stability.
In order to give the normally smooth film surface a certain degree of roughness, fine particles are normally used; in the simplest case, these particles are lightly applied to the surface (powdering process). However, the particles can also be embedded in the surface, so that micro-fine, more or less irregular protrusions occur, as a result of which the previously smooth surface receives a certain roughness profile. Embedding particles in films made of thermoplastic material can be done according to two general methods: either insoluble, primarily inorganic, fine-particle materials are added to the polymers, or substances dissolved in the polymers, for example residues of the interesterification catalyst, are specifically precipitated. U.S. Pat. No. 4,139,518 even gives a method in which both methods are combined.
An excessively roughened polyester film surface will be unsuitable as a substrate for magnetic tape applications such as video tapes. Such applications also require the film substrate to possess good abrasion resistance and durability as well as a low coefficient of friction. Abrasion resistance, durability, and good electro-magnetic conversion properties are achieved by a relatively smooth, as opposed to roughened, film surface. In order to reconcile these contrasting properties, British Pat. No. 2,087,302 describes the film which contains inert additives or catalyst residues, as a result of which numerous uneven areas occur on the surface of the film, each of these uneven areas consisting of a small protrusion and the indentation around it.
The use of inorganic particles frequently results in the formation of voids at the places where the film matrix tears away from the particle grain during orientation. When the film is subsequently manufactured into magnetic tape by coating the film with a magnetic dispersion coating, portions of the polymer covering can loosen near the surface and abrade. The quality of the film coating, as well as its electro-magnetic properties, can be negatively affected in an uncontrolled manner. The more or less wide particle size distribution of inorganic particles, as well as their tendency to form agglomerates despite pains-staking preparation, also have negative effects on the film quality.
Furthermore, the formation of internal particles by precipitating residues of the interesterification catalyst cannot be precisely controlled with regard to the quantity and size of the precipitated particles.
West German Patent Application No. P 33 13 923.7 discloses the addition of fine-particle, primarily homodisperse, crosslinked acrylic particles to the film polymer to improve the abrasion resistance of the film. The acrylic particles possess a narrow particle size distribution and are prepared by an emulsion polymerization process. Due to their chemical composition, the particles are more strongly bound to the film matrix.
The object of this invention is to manufacture a film which has a good running and abrasion behavior and at the same time, when used as a base film for magnetic storage media, has outstanding electromagnetic conversion properties.